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Having it take a full standard action is completely reasonable. If you hadn't asked, I would've just assumed that.
That being said, this is an odd case since one doesn't normally divide actions.
If the player isn't satisfied with needing to use a full action, it would make sense to rule that, with scrolls only, half a Standard action is a Minor action (or ...

The second of the Two Principles (on page 29 of the Rules Compendium) says:
Unless instructed otherwise, always round down even if the fraction is ½ or larger.
I can't find anything to instruct otherwise, so RAW is to treat it as taking no time at all.
That said, dividing actions is not something commonly done by the rules, and Hold Portal is ...

There's no such thing as "half an action", so that means you need to round it.
There are no rules anywhere that suggest that you can do something in less than an action if you use up only part of it (for example; it's not a minor action to move only half your speed, nor it is a move action to only make one of the attacks a power grants) so in this case I'd ...